1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, is concerned with a nuclear reactor core having nuclear fuel and composite burnable absorber arranged for power peaking and moderator temperature coefficient control.
2. Description of the Prior Art
In a typical nuclear reactor, such as a pressurized water reactor (PWR), the reactor core includes a large number of fuel assemblies each of which is composed of a plurality of elongated fuel elements or rods. The fuel rods each contain fissile material in the form of a stack of nuclear fuel pellets. The fuel rods are grouped together in an array which is organized to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant, such as water, is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
In the operation of a PWR it is desirable to prolong the life of the reactor core as long as feasible to better utilize the uranium fuel and thereby reduce fuel costs. To attain this objective, it is common practice to provide an excess of reactivity initially in the reactor core and, at the same time, maintain the reactivity relatively constant over its lifetime. In a PWR, initial excess reactivity is controlled primarily by use of soluble boron in the coolant water and power peaking is controlled primarily by use of burnable absorber. For long cycles, the control of initial excess reactivity by soluble boron alone would require high boron concentrations in water, which would lead to positive moderator coefficient. Therefore, in addition to power peaking control, burnable absorber is used to hold down some of the excess reactivity, so that the soluble boron concentration is appropriate to maintain the moderator temperature coefficient within the technical specifications.
In one prior art approach, a burnable absorber is mixed directly with the fissionable material of the fuel pellets and integrated therewith to enable the use of an excessive amount of fuel in the reactor core during the initial life of the fuel. In another prior art approach, a burnable absorber coating is applied to the surface of fuel pellets. For example, in U.S. Pat. No. 3,427,222 to Biancheria et al, assigned to the assignee of the present invention, the fuel pellets have a fusion-bonded coating on the surface of each pellet. Each fuel pellet is a cylindrical body composed of sintered particles of fissionable material, such as enriched uranium oxide, and an outer coating of predetermined thickness containing a burnable absorber or poison material, such as boron, cadmium, gadolinium, samarium, and europium. Examples of boron-containing compounds used are boron carbide, boron nitride and zirconium boride or zirconium diboride. The burnable absorber coating approach has been successfully applied in an integral fuel burnable absorber (IFBA) rod, manufactured and marketed by the assignee of the present invention and used in a PWR fuel assembly known commercially as the VANTAGE 5.
Up to the present, the same burnable absorber, such as zirconium diboride employed in IFBA rods, has been used for controlling both power peaking and moderator temperature coefficient. For long cycles, with high initial excess reactivity, a number of IFBA rods are used for power peaking control and oftentimes additional IFBA rods are needed for moderator temperature coefficient control. The latter is done indirectly by reducing the concentration of boron in water (used to surpress excess core reactivity) by providing for increased absorption through burnable absorber rods. This situation leads to the use of a large number of IFBA rods and a higher residual penalty.
Consequently, a need exists for a different approach to controlling both power peaking and moderator temperature coefficient than by use of a large number of IFBA rods in the nuclear reactor core as has been the practice heretofore.